1. Field of the Invention
This invention relates to a composition and method for use in inhibiting acrylate monomer polymerization at elevated temperatures.
2. Description of the Prior Art
It is well known in the art that acrylate monomers readily polymerize and that the rate of polymerization increases with increasing temperature. Common industrial methods for producing the acrylic monomers typically include purification processes such as distillation to remove impurities. However, purification operations carried out at elevated temperatures result in an increased rate of undesirable polymerization. The polymerization of acrylate monomers is undesirable because it causes fouling of processing equipment and it renders the compounds unfit for use without further treatment.
Known polymerization inhibitors for acrylates include phenothiazine, hydroquinone, the methyl ether of hydroquinone, benzoquinone, and methylene blue. Of primary interest is Japanese Pat. No. 47-18820 which discloses the use of dialkylhydroxylamine of generic structure ##STR3## (with R and R' as its alkyl radicals), singly or together with other sundry polymerization inhibitors, to inhibit polymerization of unsaturated compounds of generic structure ##STR4## (where R stands for hydrogen or methyl radical and R' and R" for alkyl radicals). Also, May in U.S. Pat. No. 3,408,422 discloses a process for stabilizing ethylenically unsaturated polyesters and a composition stabilized against premature gelation comprising (1) a hydroxy--containing ethylenically unsaturated polyester of a glycidyl polyether of a polyhydric phenol and an ethylenically unsaturated monocarboxylic acid, and (2) a hydroxylamine compound.
Phenylenediamines alone or with oxygen are known in the art as polymerization inhibitors in acrylate systems. Otsuki et al. in U.S. Pat. No. 3,674,651 discloses a process for inhibiting the polymerization of acrylic acid using a combination of diphenylamine or its derivatives and an oxygen-containing gas, or mixtures of diphenylamine or its derivatives with benzoquinone and/or hydroquinone mono-methyl-ether and an oxygen-containing gas. Wilder, in U.S. Pat. No. 4,016,198, discloses a method of inhibiting polymerization of unsaturated carboxylic acid esters and improved unsaturated carboxylic acid ester compositions comprising incorporating into the ester composition a combination of polyalkyleneamines and certain N-aryl-o or p-phenylenediamines. Also, Mullins in U.S. Pat. No. 4,017,544 discloses the use of a class of N-(nitroalkyl)-N'-phenyl-p-phenylenediamines to inhibit the polymerization of unsaturated carboxylic acid esters. Findeisen in U.S. Pat. No. 4,267,365 discloses a process for the preparation of certain oligomeric acrylic acids wherein the acrylic acid is heated in the presence of 0.001 to 1% by weight of a polymerization inhibitor consisting of molecular oxygen, nitric oxide, a phenol, a quinone, an aromatic amine, a nitro compound or diphenylpicrylhydrazyl to a temperature from about 50.degree. to 200.degree. C. Clonce et al. in U.S. Pat. No. 4,480,116 discloses an improved method for preparing and processing readily polymerizable acrylate monomers by employing phenyl-para-benzoquinone, 2,5-diphenyl-para-benzoquinone, or a mixture thereof. None of these prior art references recognizes the unique synergistic mixture of hydroxylamine and phenylenediamine or derivatives thereof having at least one N-H group as desirable for inhibiting acrylate ester polymerization.